The present invention relates to the control of soil erosion especially on the sides of rivers, drainage canals, riverbeds, levees, beaches, storm drains and the like, and more particularly relates to an erosion control system comprising a plurality of blocks, each of which is connected to the adjacent block by a connector which maintains horizontal and/or vertical block placement yet allows an articulated movement of the blocks with respect to one another allowing conformation of the overall block system to the underlying terrain, and still more particularly to blocks having horizontal hydraulic grooves for relieving hydrostatic pressure.
During the last 20 years, an industry has developed around the use of a variety of articulating concrete block matrices designed to control erosion. Most designs have the ability to conform, to varying degrees, to the settlement of soils and provide an environment for revegetation after excavation and construction of dikes, embankments and structures designed to withstand the erosive forces at the interface of water and land.
As the industry has grown, the physical properties of the devices have been tested by the Federal Highways Department and other Government departments and standard tests are being derived to assess the benefits of any system tested by these standard tests.
It is unanimously agreed in the industry and the specifying community that the principal attributes required of erosion control systems are the ability of individual blocks to remain stable during water flow in the channel or revetment profile and to relieve hydrostatic pressure under dynamic hydraulic conditions. The method and ease of installation and the degree to which the completed revetment will grow and support vegetation is a matter of choice for the end user.
The systems available today are assembled in matrix, either by connecting by means of cable or systems of interlock. See, for example, U.S. Pat. No. 4,372,705, incorporated herein by reference. The benefit of interlocking systems is that they can be laid manually. The disadvantage is, in applications that require laying in deep water, they need to be additionally cabled, incurring an additional cost of block production and installation.
The stability of the individual block is a function of the density of the concrete material, the length of the block in the direction of flow, the surface characteristics of the block and any physical connection with the adjacent blocks that would resist a turning moment. Water flows underneath the erosion control blocks, through the apertures in the blocks and between the blocks. This water flow occurs both under normal water flow conditions and when the water forms waves that break over the erosion system.
Hydrostatic pressure causes failures in concrete linings and erosion systems which do not have adequate open area. Under water flow conditions, the water flow across the top of the blocks creates a low pressure area on the surface of the blocks forming a traction force that pulls water and material up through the open areas in the revetment. The open areas permit an equalization of pressure between the upper and lower planar surfaces of the block. Thus, the erosion system must have open area to relieve the hydrostatic pressure.
Typically the erosion control blocks rest on a filter fabric. Where the lower planar surface of the block engages the fabric, the block prevents water flow through that portion of the fabric and only the remaining open areas in the filter fabric are available for relieving hydrostatic pressure. The open area of the filter fabric typically is about 6 to 8% of its area. Prior art erosion control blocks generally have vertical apertures through the block to provide approximately a 20% open area through the erosion system. Thus, the total percentage of open area which is available for the water to flow though the erosion system is about 6 to 8% of the 20%.
The surface roughness of any erosion system varies, within narrow limits, from one another. The open areas through the block due to these vertical apertures increases the roughness along the upper surface of the block. As the roughness increases, the volumetric flow of the water through the water channel or revetment is reduced thereby requiring that the water channel be made larger to handle the required volume of water flow. Thus, it is a trade off between having a sufficient number of vertical apertures through the block to provide hydraulic relief and the increased roughness of the upper surface of the block which reduces the given volume of water flow through the channel.
The density of the concrete material for the blocks is generally constant for all systems. The unit weight is somewhat limited to the amount easily handled over a normal working period by manual labor. The mutual support of the adjacent blocks varies widely with each design.
The hydraulic design attribute that differs between the various systems is the effect of this mutual support afforded by the adjoining blocks in the matrix. Whether the blocks are cabled or are interlocking, the "initial friction" between the blocks is the only force imparted by the adjacent blocks. Cables, that are in most cases in round channels, give no resistance to initial uplift of the blocks. The factor that is significant, and varies between systems, is the lineal length of surface in mutual contact as a proportion of the unit size of block. For instance a keyed or interlocked block has more contacting surface than a square block of the same height and weight. A typical problem with the dependency on this friction is the fact that a lot, and maybe most, of the applications of these systems are not on regular flat planes, there generally being humps and hollows and it is no more possible to conform to these features with fixed horizontal dimensions than to wrap a sheet of paper around a ball, without tears or creases. The practical problem for designers is that testing of the blocks are conducted in a flume with regular dimensions, yet site conditions seldom emulate the test conditions because the blocks must be cut and sized to fit the bends and abrupt changes of direction in the water channel since water channels do not have regular dimensions or directions.
It is generally accepted, that it is less expensive to manufacture block systems by means, of existing concrete masonry block plants than by wet casting. The large majority of, and the only widely available, concrete masonry production facilities utilize a production module that is designed to produce three standard concrete block masonry units. This module is typically 24 inches by 16 inches. The cost of producing an erosion control block is dominated by the production yield, in useful square area, per cycle of this production module, through the block machine. The block system that gets the most yield per cycle is going to be less expensive per unit of applied area of system. Interlocking systems generally get less yield per cycle. Cable systems that are not keyed get more yield, but require the blocks to have a number of holes running horizontally through the center. This requires special equipment and slows the production cycle time, largely off-setting the yield advantage.
To produce horizontal grooves along the bottom of the block in a horizontal production mold is difficult because it is difficult to get the blocks out of the production module. One would have to slide the blocks off the production module across the grooves requiring the block to ride up and over the forms on the bottom of the production module.
The present invention overcomes the deficiencies of the prior art.